Many housings for mechanical equipment are required to serve two generally different functions: (1) to enclose the internal components of the assembly to prevent any undesired exchange to or from the external environment; and (2) to handle large forces, torques or pressures generated internally among equipment components or transmitted to or from an external source through the housing wall.
With a conventional metal housing, both the enclosure and load bearing functions are handled by locally changing the metal from thick (load bearing) to thin (separation) by shaping the housing as a whole during manufacture. Manufacture may be by casting, forging, machining, or some other method. Generally a metal housing has a uniform composition with mechanical properties being uniform in all directions (isotropic).
Composite materials offer a much greater range of materials and properties including properties that are highly directional (anisotropic). Properties and cost are highly dependent on material variables and method of manufacture. Manufacture of composite structures by placement of plies of material leads to very strong, stiff, and light weight structures which may be very expensive, especially for hand layup. Structures made of metals can also be very strong and stiff, but tend to be heavy and in some cases prone to corrosion. Conversely lower cost composite manufacturing processes, such as molding or stamping, generally employ short fibers and do not control fiber orientation. These processes give structures with lower mechanical properties. Generally composite materials are less prone to corrosion than metals.
In the past, attempts to reduce weight and to meet structural requirements by fabricating an enclosure by a single high performance manufacturing process, for example hand layup, have resulted in high manufacturing costs and the decision not to convert to composites.